The present disclosure relates generally to printing media used with the latest digital printing technology, e.g. thermal inkjet printers, especially ones using pigmented inks. Such high quality, low cost printing equipment does not always work at an optimum level with traditional printing papers. Furthermore, issues relating to cost, image quality, environmental impact and mechanical ease of use (e.g. minimal paper jams) all must be addressed with these products. Pigmented inks make the resolution of such problems more complex because of the differences with the way pigment particles react with media in comparison with dye molecules in dye based inks. Image qualities such as ink bleed, black optical density and color saturation are greatly dependent upon the diffusion and/or interaction of pigmented ink with printing papers. Another problem that occurs is “image strike through,” when images printed on one side come through to the other side of the paper. An additional problem is extended ink dry time which refers to the time it takes for the ink to dry such that it will not smear or transfer to other surfaces.
It has been found that divalent salt, such as calcium chloride, as an additive in the surface sizing layer added during the media making process, can react with the pigments in the ink. By this means, the pigment colorant can be made to stay on the outermost layer of the media. This increases the optical density of the image on the media and reduces the dry time. However, to achieve such effects, the divalent salt needs to be used at a concentration greater than from about 6 to about 12 kg salt per ton (T) of paper. Such a high loading of chloride-containing compounds may promote drastic corrosion of the paper milling equipment used to produce the print media, and may significantly reduce the life span of the salt-contacting parts of the paper manufacturing equipment, such as, e.g., sizing rolls.
Another drawback commonly associated with the use of calcium chloride salt arises from its exothermic dissolution in water. A significant amount of heat is produced when large batches of calcium chloride salt solution is prepared, as is customary in commercial paper manufacturing processes. Solution temperatures can easily reach from about 90° C. to about 100° C. or more. The chloride-containing vapors from such a heated solution may, in some instances, cause potential environmental issues for workers involved with the mixing process. Further, the divalent salts like calcium chloride or magnesium chloride are very moisture-absorbent. The use of this type of salt can easily change the stiffness of the paper due to absorption of water into the paper. This inevitably causes some issues related to the runnability of the media in the print. These issues can cause, for example, paper jamming and/or multi-picking of the sheets from a paper tray.